Typically, a distributor in refrigeration systems receives two-phase refrigerant flow from an expansion valve and divides it equally to provide uniform feed to all circuits of an evaporator.
Each conduit of an evaporator in a refrigeration system must have an equal fluid mass flow rate of refrigerant among the conduits in order effectively to use the evaporator. For example, if during operation of the VCS with a ten-conduit capacitor, 99% of the liquid refrigerant were to flow in only two of the ten conduits, then only 20% of the evaporator's heat exchange area would be effectively utilized. A distributor is used for the purpose of rendering the mass flow to the evaporator paths uniform and thereby allow the size of the evaporator to be reduced.
Under adverse "g" conditions of the type encountered in aerospace applications, a poorly performing distributor can cause excessive cycling of the expansion valve, poor evaporator performance and compressor performance. Poor refrigerant distribution or unequal evaporator loading reduce coil capacity and contribute to flood back to the compressor.
Two-phase flow static and dynamic dividers or distributors in general for a variety of purposes, including use on refrigeration systems, have been known for some time. For instance, U.S. Pat. No. 4,085,776 shows a flow divider for liquids having solid materials suspended therein. A typical use for such a device is in the feeding of slurries to screening equipment where the liquid is to be discharged at several locations along a vibrating screen. Circular tanks were used in which the slurry was introduced tangentially in the upper portion of the tank where it underwent cyclonic mixing as it descended along the circular wall of the tank. To encourage uniform mixing, an annular flange was proposed to be located against the internal wall of the tank at a level below the inlet passages and above the discharge passages. As a result, the slurry closest to the wall was intended to move radially inwardly, with the flange producing turbulence and a mixing action in the outer regions of the liquid in the tank. In other words, distribution was achieved in this device by mixing rather than separating the phases.
U.S. Pat. No. 4,248,296 shows a distributor for mounting on the upper end of vertical condenser tubes in a falling film-type heat exchanger in which, for example, brine slurry can flow to form a falling film on the interior surfaces of the condenser tubes. A ferrule chamber includes a frusto-conical lower chamber and a spherically shaped upper chamber arranged tangentially to the frusto-conical lower chamber. One or more inlet orifices are provided in the head portion and open tangentially into the upper spherically shaped chamber to direct the fluid inwardly and downwardly into the ferrule chamber. The swirling fluid establishes an inward vortex so that a rotating, hollow cylindrical fluid film can flow down the interior surface of the associated condenser tube. Such an arrangement requires a ferrule chamber for each tube and does not concern itself with two-phase flow or an even distribution of two-phase flow to a plurality of evaporator tubes. Centrifugal action is used for wetting the condenser tubes.
Another form of two-phase flow divider is disclosed in U.S. Pat. No. 4,528,919. However, this apparatus was intended for distributing ammonia and ammonia vapor to the soil for fertilization of the soil. To accomplish this, a divider was proposed in which a fluid inlet was placed in fluid communication with two or more separate fluid outlets through fluid conduits. The multiphase fluid flowed through a fluid inlet chamber into contact with an apertured plate so that the multiphase flow could be divided in a plane perpendicular to the flow direction into multiple separate streamlets in order to flow through the fluid conduits. This apparatus was not concerned with use of the divider in adverse "g" conditions and did not propose a divider which assures even flow distribution under those conditions.
A conventional static two-phase refrigerant distributor of the type designated by the numeral 10 in FIG. 1, comprises a body or housing 11 having an inlet 12 adapted to be connected downstream of a conventional expansion valve (not shown). The body 11 is provided with a series of passages 13 (only two of which are shown) distributed evenly therearound and to which tubing 14 communicating with the heat exchanger circuits of a direct expansion evaporator (also not shown) are connected. A geometrical divider 15 having a cone shape is arranged upstream of the passages 13. A removable nozzle 16 is held in the inlet section 12 of the body 11 by a retainer ring 17. Two-phase flow was distributed at the exit of the expansion valve by impinging the flow on the geometrical flow divider 15 after passing the two phase flow through the nozzle 16.
The distributor of FIG. 1 is designed so that the liquid and vapor leaving the expansion valve enter the distributor independently. The nozzle orifice 18 increases the refrigerant velocity, thereby creating turbulence and a thorough mixing under normal "g" conditions. The mixed refrigerant continues to move at high velocity past the nozzle 16 where roughly equal proportions of the two-phase mixture are deflected by the geometrical divider 15 into each passageway 13 spaced evenly around the distributor body. The refrigerant is then conveyed by the connecting tubing 14 to each evaporator circuit.
The distributor nozzle provides high velocity and turbulence to the liquid and vapor refrigerant, key ingredients in mixing the liquid and vapor. The high velocity is accompanied by a pressure drop which causes additional liquid refrigerant to flash into vapor which increases turbulence and further homogenizes the mixture. The interchangeable nozzle permits flexibility in handling variations in evaporator applications such as load, range, evaporator temperature and different refrigerants.
This type of distributor has certain advantages. For example, it is compact and can be installed in almost any position. The interchangeable nozzle permits custom selection for any refrigerant or capacity. Air conditioning systems often employ thermostatic expansion valves with gas charged power elements. The pressure drop across the distributor in FIG. 1 provides a pressure drop to maintain the bulb colder than the diaphragm case for proper control. Furthermore, it is adaptable to any standard thermostatic expansion valve and can be applied to available multi-circuit evaporators. It must, however, always be oriented to one position, e.g. vertically, to provide proper distribution.
The jet impingement nozzle distributor is not deemed sufficient for adverse "g" conditions as are encountered in aircraft installations where a distributor will be oriented in any number of positions during the course of a flight. When the liquid refrigerant passes through the expansion valve, a portion of the refrigerant flashes into vapor resulting in a two-phase mixture at the valve outlet. By weight, the mixture is predominantly liquid; however, vapor occupies the greater volume. Thus, the liquid and vapor refrigerant tend to move at different velocities and separate into layers, with gravity pulling the heavier liquid to the bottom. Unless the distributor of FIG. 1 is maintained in a vertical position, the conduits on one side of the distributor will receive more liquid than the conduits on the other side.
Another type of distributor used in vapor cycle systems is dynamic in operation and, therefore, needlessly complex and susceptible to malfunctioning. These distributors use the general approach of distributing single phase flow rather than distributing two-phase flow. In particular, a throttle is provided upstream of each evaporator conduit path in the form of a needle or flow plate covering each conduit opening. The needles or plates are ganged together and actuated toward and away from the apertures by, for example, a linear stepper motor. In essence, each conduit has its own control valve which is actuated by feedback from some point in the flow cycle. However, the clearance between, on one hand, the needles or plates and, on the other hand, the apertures is critical in causing the refrigerant to flow equally among all the conduits. Although such a device permits the tight control of mass flow based upon its direct correlation with upstream pressure for a given flow area, the problems encountered with a dynamic system, including leaning of the needle and vibration, require careful manufacturing and adjusting procedures.